E. Hay et al., Bone morphogenetic protein-2 promotes osteoblast apoptosis through a Smad-independent, protein kinase C-dependent signaling pathway, J BIOL CHEM, 276(31), 2001, pp. 29028-29036
Bone morphogenetic protein-2 (BMP-2), a member of the transforming growth f
actor-beta (TGF-beta) family, regulates osteoblast differentiation and bone
formation. Here we show a novel function of BMP-2 in human osteoblasts and
identify a signaling pathway involved in this function. BMP-2 promotes apo
ptosis in primary human calvaria osteoblasts and in immortalized human neon
atal calvaria osteoblasts, as shown by terminal deoxynucleotidyl transferas
e-mediated nick end labeling analysis. In contrast, TGF-beta2 inhibits apop
tosis in human osteoblasts. Studies of the mechanisms of action showed that
BMP-2 increases the Bax/Bcl-2 ratio, whereas TG beta -2 has a negative eff
ect. Moreover, BMP-2 increases the release of mitochondrial cytochrome c to
the cytosol. Consistent with these results, BMP-2 increases caspase-9 and
caspase-3, -6, and -7 activity, and an anti-caspase-9 agent suppresses BMP-
2-induced apoptosis. Overexpression of dominant-negative Smad1 effectively
blocks BMP-2-induced expression of the osteoblast transcription factor Runx
2 but not the activation of caspases or apoptosis induced by BMP-2, indicat
ing that the Smad1 signaling pathway is not involved in the BMP-2-induced a
poptosis. The proapoptotic effect of BMP-2 is PKC-dependent, because BMP-2
increases PKC activity, and the selective PKC inhibitor calphostin C blocks
the BMP-2-induced increased Bax/Bcl-2, caspase activity, and apoptosis. In
contrast, the cAMP-dependent protein kinase A inhibitor H89, the p38 MAPK
inhibitor SB203580, and the MEK inhibitor PD-98059 have no effect. The resu
lts show that BMP-2 uses a Smad-independent, PKC-dependent pathway to promo
te apoptosis via a Bax/Bcl-2 and cytochrome c-caspase-9-caspase-3,-6,-7 cas
cade in human osteoblasts.